Transmission of MBMS in an OFDM communication system

ABSTRACT

The invention provides for a method of identifying a cyclic prefix to UEs in an OFDM communication system. The cyclic prefix has a dynamically variable length. The method includes, within an OFDM cell, transmitting MCCH scheduling information in a system information block in an OFDM broadcast channel, and using the MCCH scheduling information to receive the MCCH, wherein the MCCH contains MTCH scheduling information to indicate to the UE which sub-frame carries MTCH.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/522,236, filed on Jul. 6, 2009, which is a National Stage ofInternational Application No. PCT/JP2007/074367, filed on Dec. 12, 2007,which claims priority from Australian Patent Application No. 2007900103,filed on Jan. 10, 2007, the contents of all of which are incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present invention relates generally to a mobile communication systemsupporting Orthogonal Frequency Division Multiplexing, and in particularto mobile communication systems supporting proposed MultimediaBroadcast/Multicast Services.

BACKGROUND ART

As communication technology develops, services provided in mobilecommunication systems are evolving to include a multimedia broadcastcommunication service capable of supporting multimedia packet servicesto enable the transmission of large amounts of bulk data simultaneouslyto many User Equipment (UE). In order to support the multicast broadcastcommunication the 3^(rd) Generation Partnership Project (3GPP) has beenproposed a Multimedia Broadcast/Multicast Service (MBMS) in which one ormore multimedia data resources provide a service to a plurality of UE.

Proposed MBMS services can transmit the same multimedia data to aplurality of UEs through a wireless Orthogonal Frequency DivisionMultiplexing (OFDM) network. MBMS services will be able to save wirelesstransmission resources by allowing a plurality of UEs to share one radiochannel. MBMS services are intended to support the transmission of suchmultimedia data, as real-time image and voice, still images and text.

The basic time unit for the transmission and multiplexingsignals—including data, control and reference signals—in OFDM system isan OFDM symbol, which consists of a cyclic prefix (CP) followed by anuseful OFDM symbol. The useful OFDM symbol is the sum of multiplesub-carriers, each capable of carrying one modulation symbol which isreferred to as a resource element (RE) in current 3GPP standards. An REis basic frequency unit for signal transmission and multiplexing in OFDMsystem.

Current 3GPP standards specify normal and extended CP lengths forattaching to the useful OFDM symbol to avoid multipath interference atthe UE. An OFDM symbol with a normal-length CP (OFSN) can be used fortransmitting a signal requiring small or medium coverage to minimize CPoverhead. An OFDM symbol with an extended-length CP (OFSE) can be usedfor transmitting a signal which requires large coverage to avoidmultipath interference at a geographically remote UE.

When MBMS services are delivered to a single frequency network (SFN)MBMS service delivery area, the same MBMS signal is transmittedsynchronously in time, using the same frequency sub-carriers, fromwithin all cells in the MBMS service delivery area. Since an MBMS signalhas a large coverage area, including multiple cells, OFSE is normallyrequired for transmitting the MBMS signal.

3GPP normally use the term “unicast signal” to distinguish the cellspecific signal (i.e. the signal that is generally different between thecells) and “MBMS signal” which can be the same from multiple cells. TheCP length that is normally used for unicast signal transmission in acell is called default CP of that cell.

3GPP standards define a transmission unit called slot which has lengthof 0.5 ms and consist of 7 OFSNs or 6 OFSEs. A sub-frame, consists of 2slots, is currently assume to be smallest scheduling unit fortransmission and multiplexing of unicast and MBMS signal at the physicallayer. A slot or sub-frame can be viewed as a two dimensions grid (timeand frequency) of multiple RE.

3GPP standards also assume that some types of Unicast signals, such asthe L1/L2 control for Uplink (UL) scheduling, ACK for UL packagetransmission, Reference Signal for measurement, data for broadcastchannel and paging channel, synchronisation signal, etc., need to bemultiplexed with the MBMS signal in the same sub-frame. However, it isunclear how this multiplexing is done. 3GPP standards seem to assumethat RE of some types of unicast (e.g. reference signal) and MBMSsignals may be multiplexed in the same OFSE. If this is the case, it mayhappen that in a cell with normal CP being the default CP, same types ofunicast signal can be transmitted in either OFSN or OFSE at any time.Therefore, it is unclear how all UEs, receiving unicast signal, are tobe made aware of the dynamic change of CP length, between those OFDMsymbols in which only Unicast signals are transmitted and those OFDMsymbols in which MBMS and Unicast signals are multiplexed, in order toenable UEs to detect the useful part of a OFDM symbol transmitted afterthe normal or extended CP.

It would be desirable not to multiplex the MBMS signal and unicastsignal in the same OFSE. If it is not possible, it would be desirable toprovide a method of enabling a cyclic prefix length to be determined inan OFDM communication system in which the cyclic prefix length can varydynamically. It would also be desirable to provide a method of enablinga cyclic prefix length to be determined in an OFDM communication systemthat ameliorates or overcomes one or more disadvantages orinconveniences of know cyclic prefix length determination methods.

DISCLOSURE OF INVENTION

With this in mind, one exemplary aspect of the invention provides amethod of identifying a cyclic prefix to user equipments (UEs) in anorthogonal frequency division multiplexing (OFDM) communication system,the cyclic prefix having a dynamically variable length, the methodincluding:

within an OFDM cell, transmitting MBMS control channel (MCCH) schedulinginformation in a system information block in an OFDM broadcast channel;and

using the MCCH scheduling information to receive the MCCH, wherein theMCCH contains MBMS transport channel (MTCH) scheduling information toindicate to the UE which sub-frame carries MTCH.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an OFDM communication system;

FIG. 2 is a schematic diagram illustrating the inclusion of a cyclicprefix in OFDM symbols transmitted in the OFDM communication system ofFIG. 1;

FIG. 3 is a schematic diagram showing a sub frame structure for use withthe OFDM communication system in FIG. 1;

FIG. 4 is a schematic diagram showing the interrelationship betweenvarious channels used for transmission of information in the OFDMcommunication system of FIG. 1;

FIG. 5 is a schematic diagram illustrating selected functionalcomponents of an exemplary transmitter and UE forming part of the OFDMcommunication system of FIG. 1;

FIG. 6 is a timing diagram showing the temporal positions of correlationpeak signals during operation of the OFDM communication system of FIG.1;

FIG. 7 is a timing diagrams showing the temporal positions ofcorrelation peak signals during operation of the OFDM communicationsystem of FIG. 1;

FIG. 8 is a schematic diagram illustrating selected functionalcomponents of an exemplary transmitter and UE forming part of the OFDMcommunication system of FIG. 1;

FIG. 9 is a timing diagram showing the temporal positions of correlationpeak signals during operation of the OFDM communication system of FIG.1;

FIG. 10 is a schematic diagram showing an alternate sub frame structurefor use with the OFDM communication system in FIG. 1;

FIG. 11 is a timing diagram showing the temporal positions ofcorrelation peak signals during operation of the OFDM communicationsystem of FIG. 1; and

FIG. 12 is a timing diagram showing the temporal positions ofcorrelation peak signals during operation of the OFDM communicationsystem of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention will now be describedwith reference to the attached drawings. It is to be understood thatthese embodiments are exemplary only and are not intended to limit thegenerality of the invention described previously.

Referring now to FIG. 1, there is shown generally an OFDM communicationnetwork 10 for provision of MBMS services. A content provider 12provides multimedia content to a Broadcast Multimedia Service Centre(BM-SC) 14. That multimedia content is transmitted to a plurality ofAccess Gateways (AGWs) 16 and 18 via communications interfaces 20 and22. The AGWs 16 and 18 form part of an Evolved Universal TerrestrialRadio Access Network (E-UTRAN) 24. The AGWs 16 and 18 distribute themultimedia content to E-UTRAN nodes (eNBs) 26 to 36, each of whichprovides for radio transmission of the multimedia content within aseparate cell. One or more cells defining a zone within which a sameMBMS service is delivered is called an MBMS service delivery area. UEs38 and 40 are able to received MBMS services within MBMS servicedelivery areas.

Data is transmitted within the OFDM communication system 10 within aseries of sub frames. Part of a representative sub-frame 50 is shown inFIG. 2. The sub-frame includes a series of OFDM symbols, with each OFDMsymbol including a CP 52 and a useful part 54. Each OFDM symbol is a sumof series of sub-carriers, exemplary ones of which are referenced 56,58, 60 and 62. The temporal length T_(symbol) of the entire OFDM symbolequals the sum of the temporal length T_(CP) of the CP plus the temporallength T of the useful OFDM symbol.

The CP is included in each OFDM symbol in order to maintainorthogonality between sub-carriers. The CP is a copy of the last portionof the useful part of the OFDM symbol appended to the front part of thesymbol during a guard interval. Multiple path transmission of the OFDMsymbol on the sub-carriers 56 to 62 causes tones and delayed replicas oftones to arrive at the UEs with some delay spread. This leads to amisalignment and loss of orthogonality between the tones. The CP allowsthe tones to be realigned at the UEs, thus regaining orthogonality. TheCP length is chosen to eliminate inter-signal interference since theamount of time dispersion from the channel is smaller than the durationof the CP. Although the amount of the overhead increases as the CP getslonger, the CP must nevertheless be long enough to account for theanticipated multipath delay spread experienced by the system.

FIG. 3 is a schematic diagram showing sub-frame structures that arecurrently proposed by the 3GPP standardisation group. A first sub-framestructure 70 includes 14 OFDM symbols transmitted on a plurality ofsub-carriers. The single data symbol transmitted by each sub-carrierduring each OFDM symbol is known as a resource element (RE). Thesub-frame structure 70 is intended to carry Unicast signals only andinclude a CP of normal length.

However, the sub frame 72 has also been proposed to carry both Unicastand MBMS signals which are multiplexed in a same sub-frame. The subframe structure 72 is transmitted over the same length of time as thesub-frame structure 70 (namely, 1 ms). However, since MBMS services canbe received at the UEs from relatively distant cells, each symbol in thesub-frame structure 72 is required to include a CP of extended length toensure that the received MBMS signal is not impacted by multi-pathinterference. By way of example, a normal-length CP may have a length of10 data samples in a 1.25 MHz bandwidth system whereas an extendedlength CP may have a length of 32 data samples in a 1.25 MHz bandwidthsystem. REs forming part of the first three symbols of the sub-framestructure 72 are defined in the current 3GPP standards as being for usewith Unicast signals only, whereas REs forming part of the remainingnine symbols of the sub-frame structure 72 are defined as being able tocarry both Unicast and MBMS signals.

However, it is not yet known how all UEs receiving Unicast signals froma cell for data reception and/or measurement purposes are to be madeaware of a possible dynamic change of CP length for received sub-frames.This problem occurs, for example, when the default CP length within aparticular cell is a normal-length CP but the cell also forms part of aMBMS service delivery area which requires an extended-length CP to beused during the transmission of all symbols carrying MBMS data. UEscurrently have no way of knowing whether the CP length of the OFDMsymbols in any given sub-frame is normal or extended.

In order for a UE to be able to determine whether a CP transmitted inany given OFDM sub-frame has a normal length or extended length, an MBMScontrol or scheduling channel (MCCH or MSCH) can be designed so that UEscan read information from the MCCH or MSCH characterising whichsub-frame in an MBMS transport channel (MTCH) contains MBMS data (andtherefore uses an extended-length CP).

As can be seen in FIG. 4, a special system information block (SIB) 80can be included in a conventional OFDM cell specific Broadcast Channel(BCH) to indicate to the UEs the location of information transmitted inan MBMS Control (Signalling) Channel (MCCH) 82 which identifies whichsub-frame will carry MTCH and thus the length of the CP used in thesub-frames within an MBMS service delivery area. Exemplary sub-frames 84to 94 are shown in FIG. 4. Sub-frames 84, 86 and 90 include Unicastsymbols only and hence use a normal length CP, whilst sub-frames 86, 92and 94 multiplex both MBMS and Unicast symbols and use an extendedlength CP.

Advantageously, no additional complexity is required for UE PhysicalLayer processing is required in this scenario, although UE must be ableto receive not only the OFDM cell specific BCH but also the MBMS MCCHregardless of whether a particular UE wishes to receive an MBMS service.

An alternative to the above-described techniques is illustrated in FIG.10. This figure shows a first new sub-frame structure 160 and a secondnew sub-frame structure 180 for use when MBMS and Unicast signals aremultiplexed in the same sub-frame and transmitted from a cell withnormal-length and extended-length CP, as default CP, respectively. Inthe new sub-frame 160, the first n (typically 2 or 3) OFDM symbols 162always use a normal-length CP for transmitting Unicast signals only(i.e. mainly L1/L2 control for UL, RS and ACK) and the remaining OFDMsymbols 164 use an extended-length CP for transmitting mainly MBMSsignals.

In the new sub-frame 180, the first n (typically 2 or 3) OFDM symbols182 always use a extended-length CP for transmitting Unicast signalsonly (i.e. mainly L1/L2 control for UL, RS and ACK) and the remainingOFDM symbols 184 use an extended-length CP for transmitting mainly MBMSsignals.

The new sub-frames 160 and 180 should be such that the number of OFDMsymbols used for MBMS must be the same and each of these symbols mustend at the same time.

Advantageously such a new sub-frame structure 160 enables the UE toreceive the L1/L2 control transmitted in the first n OFDM symbols in asub-frame without being required to know if the MBMS signal istransmitted in the sub-frame or not since the first n OFDM symbolsalways use a normal-length CP. Moreover, if the UE only uses the RS inthe 1^(st) OFDM symbol in a slot (there are two slots in a sub-frame)for measurement, then UE does not need to be aware of whether MBMS isbeing transmitted in the sub-frame or not. If the sub-frame carrying theBCH always use default CP length then UE does not need to be aware ofwhether MBMS is being transmitted in the sub-frame or not.

As another alternative to the above described technique, or as acompliment thereto when the UE misses or cannot read the MCCH and UEstill need to know the CP length used in all OFDM symbols of asub-frame, the UE can perform blind detection of the CP length. Thisalternative is only required when the UE receives a unicast signal froma cell with normal-length CP as default. FIG. 5 is a schematic diagramshowing selected functional blocks of a transmitter 100 and receiver(UE) 102 in which CP length blind detection is performed. Followingmodulation and time/frequency mapping in block 104, an Inverse PastFourier Transform (IFFT) is performed in the transmitter at IFFT block106 in order to transform frequency domain data into the time domainsignal. CP insertion block 108 is then used to introduce the cyclicprefix into OFDM symbols in order to avoid inter-symbol interference atthe UE and RF block 110 is then used to transmit the signal over channel112.

At the UE 102, a corresponding RF block 114 is used to receive anddigitise the signal transmitted over the channel 112. A CP lengthdetection block 116 then detects the length of the CP used in a givenOFDM sub-frame. Thereafter, the CP is eliminated by CP removal block 118and the useful part of the OFTM symbol is converted from the time domaininto the frequency domain by FFT block 120, prior to time/frequencyde-mapping and demodulation by processing block 122.

The UE 102 uses a replica of a Reference Signal (RS) transmitted duringthe 1^(st) symbol in an OFDM sub-frame for cross-correlation with areceived signal output from the RF block 114. The replica RS is storedin memory 124 and output through an IFFT block 126 that is functionallyidentical to the IFFT block 106 in the transmitter 100. The output ofthe IFFT block 126 is correlated with the output of the RF block 114 bya correlator 128, and the length of the CP is detected by the CPdetection block 130.

FIG. 6 shows selected data samples in an exemplary 1^(st) OFDM symbol ina sub-frame 140 as well as a first theoretical correlation peak P1occurring at the start of the useful part of the OFDM symbol where anormal length CP is used, and a second theoretical correlation peak P2occurring at the start of the useful part of the OFDM symbol where anextended length CP is used. If the UE 102 can always correctly estimatethe temporal position T_ref of the start of each symbol, then the UE 102simply needs to compare the two correlation values P1 and P2 in order todecide which CP length to use.

There is always error in estimations of the temporal position T_ref ofthe start of each symbol, and accordingly two windows W1 and W2 arerequired by the UE 102 within which to search for correlation signalpeaks. The maximum window size MW is given by the difference in temporalpositions between the peaks P1 and P2, which can be seen as the maximummargin of error for the estimation of the temporal position T_ref of thestart of each sub-frame.

However, due to the fact that the temporal spacing between transmissionsof RS is typically 6 sub-carriers, extra correlation peaks occur in theprofile shown in FIG. 6. A first extra correlation peak EP1 resultingfrom use of a normal length CP, as well as a second extra correlationpeak EP2 resulting from use of an extended length CP, are also shown inthis figure. As can be seen in Table 1, depending up the bandwidth ofthe OFDM communication system, the correlation peak EP2 may be spacedfrom the correlation peak P1 (and correlation peak EP1 may be spacedfrom the correlation peak P2) by only 1 sample. This leaves a very smallpractical window PW to account for the T_ref estimation error.

TABLE 1 (Assuming T_ref = 0) System (MHz) 1.25 2.5 5 10 15 20 P1 (sampleposition) 10 20 40 80 120 160 P2 (sample position) 32 64 128 256 384 512MW = (P2 − P1) 22 44 88 176 264 352 (no. of samples) EP2 (sample 11 2243 86 128 171 position) PW = (EP2 − P1) 1 2 3 6 8 11 (no. of samples)

A first way of addressing this issue is to use to different RS sequences(referenced RS1 and RS2 in FIG. 5) in the 1^(st) OFDM symbol in asub-frame where an MBMS signal is able to be transmitted and where anMBMS signal is not able to be transmitted. As seen in FIG. 7, the UE 102perform cross correlation of the received signal with each of thereplica of RS1 and RS2 to generate two profiles. The CP length can thenbe determined from whichever of the two profiles has strongercorrelation peak.

A second way of addressing this issue is to increase the distancebetween the correlation peaks P1 and EP2 (as well as the distancebetween EP1 and P2). One manner to achieve this separation is byapplying a predefined non-zero cyclic delay (CD) to the OFDM symbols inthe sub-frame where an MBMS signal is able to be transmitted andapplying zero CD to the OFDM symbols in the sub-frame where an MBMSsignal is not able to be transmitted. FIG. 8 is a schematic diagramshowing selected functional blocks of a transmitter 150 and receiver(UE) 152 which are functionally identical to the transmitter 100 andreceiver (UE) 102 shown in FIG. 5, except for the addition in thetransmitter 150 of a CD insertion block 154 between the IFFT block 106and the CP insertion block 108 and the storage of only a single RSsequence corresponding to the zero CD RS sequence in the memory block124 of the UE 152.

In operation, the UE 152 performs one correlation using the zero CD RSsequence to generate a single correlation peak profile, and thendetermines the CP length based upon which window (i.e. either the windowW1 centred on the beginning of the useful part of the OFDM symbol when anormal-length CP is used, or the window W2 centred at about CD samplesfrom the centre of W1) has the strongest correlation peak.

As seen in FIG. 9, the effect of applying a predefined non-zero CD tothe OFDM symbols in the sub-frame where an MBMS signal is transmittedand applying zero CD to the OFDM symbols in the sub-frame where an MBMSsignal is not transmitted causes a separation of the correlation peaksP1 and EP2. In principle, in the sub-frame where an MBMS signal istransmitted, a non-zero CD only needs be applied on the RS in the 1^(st)OFDM symbol. However, to simplify system implementation and to make CDmore transparent to the UE, it may be desirable that non-zero CD beapplied to all OFDM symbols in a given sub-frame.

Table 2 below shows an example where a non-zero CD is used. It can beseen that the practical window PW is significantly larger than in Table1.

TABLE 2 System, MHz 1.25 2.5 5 10 15 20 P1 10 20 40 80 120 160 CD 8 1735 71 107 143 EP2 19 39 78 157 235 314 PW = (EP2 − P1) 9 19 38 77 115154

Yet another alternative to the above-described techniques provided by acombination of blind CP length detection, as described in relation toFIGS. 5 to 9, and the new sub-frame structure 160 shown in FIG. 10. Inthis alternative embodiment, the new sub-frame structure 160 is used fortransmission of MBMS signals.

Two variations exist for this alternative embodiment. In a first ofthese variations, two different RS sequences are used in the 1^(st) OFDMsymbols in the sub-frame where Unicast signals only are transmitted orwhere Unicast or MBMS signals are transmitted. In operation, the UEreceives Unicast signals in the first n OFDM symbols knowing that anormal-length CP is used. During the 1^(st) OFDM symbol, the UE detectswhich of the RS sequences is used as shown in FIG. 11. The UE then usesthe detected RS for decoding of other Unicast data channel such as L1/L2control channel.

In a second variation, the same RS sequence is used in the 1^(st) OFDMsymbols in the sub-frame where only Unicast signals or MBMS signals aretransmitted. In the sub-frame where MBMS is transmitted, a CD will beset to a non-zero value during the first n OFDM symbols. In operation,the UE receives Unicast signals in the first n OFDM symbols knowing thata normal-length CP is used. The UE can decode other Unicast datachannels such as L1/L2 control channel without any delay. During the1^(st) OFDM symbol, the UE detects if a zero CD or a non-zero CD hasbeen applied to the RS as shown in 12.

In principle, in the sub-frame where MBMS signal is transmitted, anon-zero CD only need be applied on the RS in the 1^(st) OFDM symbol.However, to simplify system implementation and to make cyclic delaytransparent to the UE receiving L1/L2 control channel, it may bedesirable that non-zero CD is applied to the first n OFDM symbols in thesub-frame.

These two variations advantageously enable the UE to receive L1/L2control transmitted in the first n OFDM symbols in a sub-frame withoutbeing required to know if the MBMS signal is transmitted in thesub-frame or not. Moreover, the UE can detect the CP length used in n+1,n+2, . . . OFDM symbols. CP length detection at the UE involves no ornegligible increase in delay of L1/L2 control channel decoding.

It is to be understood that modifications or additions may be made tothe above described embodiments without departing from the spirit orambit of the invention as defined in the claims appended hereto.

Another exemplary aspect of the invention provides a method ofidentifying a cyclic prefix length in an OFDM communication system, thecyclic prefix having a dynamically variable length, the methodincluding:

correlating a stored replica of one or more reference signals able to betransmitted in the k-th OFDM symbol in a sub-frame with a receivedsignal to generate one or more correlation profiles;

The k-th may be the first OFDM symbol in a sub-frame

In the case of only one correlation profile being generated, the methodmay further includes the steps of:

detecting a correlation signal peak temporally proximate a firstpossible beginning of a useful part of a hypothetical OFDM symbol shouldit include a normal-length cyclic prefix, and a second possiblebeginning of the useful part of the hypothetical OFDM symbol should itinclude an extended-length cyclic prefix; and

determining the length of the cyclic prefix from the strength of thedetected correlation peaks.

In one exemplary embodiment, a first reference signal is transmitted ina sub-frame carrying MBMS signals and a second reference signal istransmitted in a sub-frame not carrying MBMS signals, the first andsecond reference signals being different from each other, the methodfurther including the steps of:

separately correlating stored replicas of the first and second referencesignals with the received signal to respectively produce a first and asecond correlation profile; and

determining the cyclic prefix length from whichever of the first andsecond profiles results in the strongest correlation peaks.

In another exemplary embodiment, the method further includes:

processing the OFDM symbol prior to transmission to introduce aseparation between the two possible correlation signal peaks.

The separation may be introduced by applying a non-zero cyclic delay toOFDM symbols in a sub-frame in which an extended-length cyclic prefix isused, and by applying zero cyclic delay to OFDM symbols in a sub-framein which an extended-length cyclic prefix is not used.

Another exemplary aspect of the invention provides a method ofidentifying a cyclic prefix in an OFDM communication system, the cyclicprefix having a dynamically variable length, the method including:

within an OFDM cell, transmitting MCCH scheduling information schedulinginformation in a system information block in an OFDM broadcast channel;and

using the MCCH scheduling information to receive the MCCH, wherein theMCCH contains MTCH scheduling information to indicate to the UE whichsub-frame carries MTCH; and

where the MBMS point-to-multipoint control channel is missed or unableto be read, correlating a stored replica of one or more referencesignals able to be transmitted in an OFDM symbol with a received signal;

detecting a correlation signal peak temporally proximate a firstpossible beginning of a useful part of a hypothetical OFDM symbol shouldit include a normal-length cyclic prefix, and a second possiblebeginning of the useful part of the hypothetical OFDM symbol should itinclude an extended-length cyclic prefix; and

determining the length of the cyclic prefix from the strength of thedetected correlation peaks.

Another exemplary aspect of the invention provides a method of OFDM datatransmission in a sub-frame of m OFDM symbols in which the cyclic prefixlength of each symbol can vary dynamically, the method including thesteps of:

including a normal-length cyclic prefix in a first n OFDM symbols in asub-frame, n being an integer having a value less m, and including anextended-length cyclic prefix in the remaining m−n OFDM symbols, whereinMBMS signals are transmitted in the remaining m−n OFDM symbols.

The value of n may be 3 or less.

Another exemplary aspect of the invention provides a method of cyclicprefix length detection in which data is transmitted as described above,the method of cyclic prefix length detection including the step of:

correlating a stored replica of one or more reference signals able to betransmitted in the k-th OFDM symbol transmitted in a sub-frame with areceived signal.

The k-th symbol may be the first OFDM symbol in a sub-frame

In one exemplary embodiment, a first reference signal is transmitted ina sub-frame carrying MBMS signals and a second reference signal istransmitted in a sub-frame not carrying MBMS signals, the first andsecond reference signals being different from each other, the methodfurther including the steps of:

separately correlating stored replicas of the first and second referencesignals with the received reference signal to respectively produce afirst and a second correlation profile; and

determining the length of the cyclic prefix used in the remaining m−nsymbols from whichever of the first and second profiles results in thestrongest correlation peaks.

In another exemplary embodiment, the method further includes the stepof:

processing the first OFDM symbol prior to transmission to introduce aseparation between the two possible correlation signal peaks.

The separation may be introduced by applying a non-zero cyclic delay toeach of the first n OFDM symbols in a sub-frame in which anextended-length cyclic prefix is used, and by applying zero cyclic delayto each of the first n OFDM symbols in a sub-frame in which anextended-length cyclic prefix is not used.

This application is based upon and claims the benefit of priority fromAustralian patent application No. 2007900103, filed on Jan. 10, 2007,the disclosure of which is incorporated herein in its entirety byreference.

The invention claimed is:
 1. A base station for orthogonal frequencydivision multiplexing (OFDM) data transmission in a plurality ofsub-frames, the base station comprising: a transmitter which transmits adefault cyclic prefix for each of first one or more OFDM symbols in eachof the plurality of sub-frames, the number of said first one or moreOFDM symbols being integer n having a value less than m, where m is anumber of OFDM symbols in each of the plurality of sub-frames, and anextended-length cyclic prefix for remaining m-n OFDM symbols, wherein amultimedia broadcast multicast service (MBMS) signal is transmitted inthe remaining m-n OFDM symbols, and wherein n is 2 or 3, and wherein alength of the default cyclic prefix is less than a length of theextended-length cyclic prefix.
 2. The base station according to claim 1,wherein the default cyclic prefix comprises a cyclic prefix that is usedfor unicast signal transmission.
 3. The base station according to claim1, wherein a cyclic prefix length for each OFDM symbol variesdynamically.
 4. A user equipment for orthogonal frequency divisionmultiplexing (OFDM) data reception in a plurality of sub-frames, theuser equipment comprising: a receiver which receives a default cyclicprefix for each of first one or more OFDM symbols in each of theplurality of sub-frames, the number of said first one or more OFDMsymbols being integer n having a value less than m, where m is a numberof OFDM symbols in each of the plurality of sub-frames wherein each ofthe plurality of sub-frames includes an extended-length cyclic prefixfor remaining m-n OFDM symbols, wherein a multimedia broadcast multicastservice (MBMS) signal is transmitted in the remaining m-n OFDM symbols,and wherein n is 2 or 3 and wherein a length of the default cyclicprefix is less than a length of the extended-length cyclic prefix. 5.The user equipment according to claim 4, wherein the default cyclicprefix comprises a cyclic prefix that is used for unicast signaltransmission.
 6. The user equipment according to claim 4, wherein acyclic prefix length for each OFDM symbol varies dynamically.
 7. Acommunications method implemented in a base station of orthogonalfrequency division multiplexing (OFDM) data transmission in a pluralityof sub-frames, the communications method comprising: transmitting, by atransmitter of the base station, a non-multimedia broadcast multicastservice (non-MBMS) signal in first one or more OFDM symbols in each ofthe plurality of sub-frames, the number of said first one or more OFDMsymbols being integer n having a value less than m, where m is a numberof OFDM symbols in each of the plurality of sub-frames, and an MBMSsignal in remaining m-n OFDM symbols, wherein a default cyclic prefix istransmitted for the non-MBMS signal and an extended-length cyclic prefixis transmitted for the MBMS signal, and wherein n is 2 or 3 and whereina length of the default cyclic prefix is less than a length of theextended-length cyclic prefix.
 8. The communications method according toclaim 7, wherein the default cyclic prefix comprises a cyclic prefixthat is used for unicast signal transmission.
 9. The communicationsmethod according to claim 7, wherein a cyclic prefix length for eachOFDM symbol varies dynamically.
 10. The communications method accordingto claim 7, wherein the non-MBMS signal comprises a unicast signal. 11.A communications method implemented in a user equipment of orthogonalfrequency division multiplexing (OFDM) data reception in a plurality ofsub-frames, the communications method comprising: receiving, by areceiver of the user equipment, a non-multimedia broadcast multicastservice (non-MBMS) signal in first one or more OFDM symbols in each ofthe plurality of sub-frames, the number of said first one or more OFDMsymbols being integer n having a value less than m, where m is a numberof OFDM symbols in each of the plurality of sub-frames, wherein an MBMSsignal is transmitted in remaining m-n OFDM symbols, wherein a defaultcyclic prefix is transmitted for the non-MBMS signal and anextended-length cyclic prefix is transmitted for the MBMS signal, andwherein n is 2 or 3 and wherein a length of the default cyclic prefix isless than a length of the extended-length cyclic prefix.
 12. Thecommunications method according to claim 11, wherein the default cyclicprefix comprises a cyclic prefix that is used for unicast signaltransmission.
 13. The communications method according to claim 11,wherein a cyclic prefix length for each OFDM symbol varies dynamically.14. The communications method according to claim 11, wherein thenon-MBMS signal comprises a unicast signal.
 15. A communications systemfor orthogonal frequency division multiplexing (OFDM) data transmissionin a plurality of sub-frames, the communications system comprising: auser equipment; a base station comprising a transmitter which transmits,to the user equipment, a default cyclic prefix for each of first one ormore OFDM symbols in each of the plurality of sub-frames, the number ofsaid first one or more OFDM symbols being integer n having a value lessthan m, where m is a number of OFDM symbols in each of the plurality ofsub-frames, and an extended-length cyclic prefix for remaining m-n OFDMsymbols, wherein a multimedia broadcast multicast service (MBMS) signalis transmitted in the remaining m-n OFDM symbols, and wherein n is 2 or3 and wherein a length of the default cyclic prefix is less than alength of the extended-length cyclic prefix.
 16. A communications methodimplemented in a communications system of orthogonal frequency divisionmultiplexing (OFDM) data transmission in a plurality of sub-frames, thecommunications method comprising: transmitting, by a transmitter of abase station, a non-multimedia broadcast multicast service (non-MBMS)signal in first one or more OFDM symbols in each of the plurality ofsub-frames, the number of said first one or more OFDM symbols beinginteger n having a value less than m, where m is a number of OFDMsymbols in each of the plurality of sub-frames, and an MBMS signal inremaining m-n OFDM symbols, wherein a default cyclic prefix istransmitted for the non-MBMS signal and an extended-length cyclic prefixis transmitted for the MBMS signal, and wherein n is 2 or 3 and whereina length of the default cyclic prefix is less than a length of theextended-length cyclic prefix.
 17. A base station for orthogonalfrequency division multiplexing (OFDM) data transmission in a pluralityof sub-frames, the base station comprising: transmission means fortransmitting a default cyclic prefix for each of first one or more OFDMsymbols in each of the plurality of sub-frames, the number of said firstone or more OFDM symbols being integer n having a value less than m,where m is a number of OFDM symbols in each of the plurality ofsub-frames, and an extended-length cyclic prefix for remaining m-n OFDMsymbols, wherein a multimedia broadcast multicast service (MBMS) signalis transmitted in the remaining m-n OFDM symbols, and wherein n is 2 or3 and wherein a length of the default cyclic prefix is less than alength of the extended-length cyclic prefix.
 18. A user equipment fororthogonal frequency division multiplexing (OFDM) data reception in aplurality of sub-frames, the user equipment comprising: receiving meansfor receiving a default cyclic prefix for each of first one or more OFDMsymbols in each of the plurality of sub-frames, the number of said firstone or more OFDM symbols being integer n having a value less than m,where m is a number of OFDM symbols in each of the plurality ofsub-frames, wherein each of the plurality of sub-frames includes anextended-length cyclic prefix for remaining m-n OFDM symbols, wherein amultimedia broadcast multicast service (MBMS) signal is transmitted inthe remaining m-n OFDM symbols, and wherein n is 2 or 3 and wherein alength of the default cyclic prefix is less than a length of theextended-length cyclic prefix.
 19. A base station for orthogonalfrequency division multiplexing (OFDM) data transmission in a pluralityof sub-frames, comprising: transmission means for transmitting anon-multimedia broadcast multicast service (non-MBMS) signal in firstone or more OFDM symbols in each of the plurality of sub-frames, thenumber of said first one or more OFDM symbols being integer n having avalue less than m, where m is a number of OFDM symbols in each of theplurality of sub-frames, and an MBMS signal in remaining m-n OFDMsymbols, wherein a default cyclic prefix is transmitted for the non-MBMSsignal and an extended-length cyclic prefix is transmitted for the MBMSsignal, and wherein n is 2 or 3 and wherein a length of the defaultcyclic prefix is less than a length of the extended-length cyclicprefix.
 20. A user equipment for orthogonal frequency divisionmultiplexing (OFDM) data reception in a plurality of sub-frames,comprising: receiving means for receiving a non-multimedia broadcastmulticast service (non-MBMS) signal in first one or more OFDM symbols ineach of the plurality of sub-frames, the number of said first one ormore OFDM symbols being integer n having a value less than m, where m isa number of OFDM symbols in each of the plurality of sub-frames, whereinan MBMS signal is transmitted in remaining m-n OFDM symbols, wherein adefault cyclic prefix is transmitted for the non-MBMS signal and anextended-length cyclic prefix is transmitted for the MBMS signal, andwherein n is 2 or 3 and wherein a length of the default cyclic prefix isless than a length of the extended-length cyclic prefix.